Summary:
EnvZ is the sensory histidine kinase of the EnvZ/OmpR two-component system which regulates expression of the major outer membrane porin genes, ompF and ompC, in response to fluctuations in extracellular osmotic pressure.

EnvZ undergoes autophosphorylation by transferrring the gamma phosphate of ATP to histidine residue 243. The phosphate is then transferred to asp55 of the OmpR response regulator [Forst89]. Phosphorylated OmpR serves as a transcription factor, differentially modulating expression of the outer membrane porin genes [Matsuyama86, Mizuno87]. EnvZ also possesses an OmpR-phosphate phosphatase function [Forst89]. The specific mechanism by which EnvZ senses changes in osmolarity is not yet understood.

The envZ ompR genes are transcribed as a single polycistronic mRNA however the amount of EnvZ molecules per cell is significantly less than OmpR during exponential growth in medium with varying osmolarities [Cai02a]. EnvZ is a dimeric, inner membrane protein [Yaku97, Forst87] containing two trans-membrane regions, a periplasmic domain and a cytoplasmic domain - the latter including a 228 residue kinase/phosphatase domain. NMR solution structures of the active portions of the cytoplasmic domain of EnvZ have been reported [Tomomori99, Tanaka98a].